This invention relates to an apparatus and a method for smoothing the acceleration and deceleration characteristics of an electric motor and, more particularly, to an apparatus and a method for reducing the brush pressure against the commutator of an electric motor during low speed operation.
In many fields, particularly the field of centrifugation, it is desirable that the electric motor used to drive an apparatus has smooth acceleration and deceleration characteristics at low speeds. This problem is particularly acute in centrifuging materials. For example, in density gradient separations, fluids of different densities are placed in layers, with the highest density fluid forming the bottom layer, in a test tube. Particles to be separated are placed on top of the fluid layers. The test tube is then placed in a centrifuge and the particles subjected to centrifugal force.
If the acceleration of the rotor is uneven, the separation between the several density layers will be disturbed with the result that fractionation of the particles may not be as precise as desired. Similarly, as the density gradient is decelerated following separation, particularly as the lower speeds are approached, the same discontinuities and abrupt stops can distort the gradient, i.e., mix the density layers and reduce the precision of the fractionation. Also, if the rate of acceleration and/or deceleration is too high, particularly at low speeds, similar distortions of the separation between density layers can occur.
Similar problems can arise in most types of centrifugation work, whether it be using swinging-buckets, angle rotors, vertical rotors or even continuous centrifuges. In all of these cases, the fractionation or separation of the components can be hampered both during starting and stopping of the centrifuge rotor if the acceleration and deceleration is uneven, too great, irregular or discontinuous.
These problems encountered during acceleration and deceleration at low speeds are brought about typically because of the use of brush type electric motors. No matter how much care is taken in building a motor's commutator, it is never precisely round and there are, of course, gaps or discontinuities between commutator segments. To maintain proper brush contact at operating speeds, relatively high forces are applied to the brushes. Unfortunately at low speeds, typically below 1000 revolutions per minute (rpm), this high brush force causes the rotor to pulsate or turn irregularly. In fact, the motor may be brought to an abrupt stop by the brushes "hanging up" on a particular commutator segment or gap. There is another similar effect caused by variation in the electrical current through the motor known as magnetic cogging. This effect, however, is relatively insignificant in comparison to the frictional effects.
It is known in the prior art to provide an apparatus for totally lifting the brushes from contact with the commutator to provide a long term friction-free coasting. Patents that are descriptive of this type technique are U.S. Pat. Nos. 3,278,777 and 2,128,598. While the teaching of these patents, if applied to the problem at hand would alleviate to some extent the deceleration problem, they would not be at all useful for solving the acceleration problem since they could not contact the commutator and hence could not apply power to the motor.
It is therefore an object of this invention to obviate many of the vagaries inherent in the prior art brush type electric motor acceleration and deceleration characteristics.
Another object of this invention is to provide an improved apparatus for smoothing the acceleration and deceleration characteristics of brush type electric motors, particularly at low speeds.